PHP can be installed with a large number of different (optional) extensions. Curl is one of those extensions - and it is normally included and enabled - it provides the curl_init() function. If your php installation doesn't have that extension you'll get the above error. For more details see
http://www.php.net/manual/en/ref.curl.php. To see what extensions you have, set up a page with
<?php phpinfo(); ?>
DokuWiki has its own HTTP client. Try emailing, Esther, the plugin author and seeing if she will change from using curl to using DW own http client. There is a google group for the plugin (search for "tag plugin"). I'd paste the link only google is being unresponsive.